An FFV internal combustion engine that is capable of using a fuel in which different kinds of fuel are mixed is known as an internal combustion engine for an automobile. A fuel which contains a mixture of an alcohol such as ethanol and a hydrocarbon-based fuel such as gasoline is a typical mixed fuel that is used by an FFV internal combustion engine. A dual-injection internal combustion engine is also known which is equipped with a port injection valve that injects fuel into an intake port and an in-cylinder injection valve that injects fuel directly into a cylinder, and which can change an injection ratio between the two fuel injection valves in accordance with the operating state. Further, Japanese Patent Laid-Open No. 2006-214415 discloses technology relating to a dual-injection internal combustion engine for an FFV that combines the two features of the above described two kinds of internal combustion engines. That is, the technology disclosed in Japanese Patent Laid-Open No. 2006-214415 relates to an internal combustion engine that is equipped with an in-cylinder injection valve and a port injection valve and that is capable of injecting a mixed fuel (in the example in the aforementioned publication, an alcohol-mixed fuel) from both fuel injection valves.
In the conventional FFV internal combustion engine that uses an alcohol-mixed fuel, engine control is performed by a method that is in accordance with the alcohol concentration of the fuel. More specifically, because the heating value per unit volume decreases as the alcohol concentration of fuel increases, the fuel injection amount is determined according to the alcohol concentration of the fuel. The reason for this is that if the fuel injection amount is excessive, emissions will increase, while if the fuel injection amount is insufficient, the drivability will deteriorate. Further, the higher that the alcohol concentration of the fuel is, the faster that the combustion rate will be, and the exhaust gas temperature is liable to decrease by a corresponding amount. Consequently, the ignition timing is determined in accordance with the alcohol concentration of the fuel. This is because the exhaust gas temperature is increased by retarding the ignition timing, and as a result emissions are reduced. However, there is also the contrary situation that if the ignition timing is retarded too much, the drivability will deteriorate. For such reasons, in the conventional FFV internal combustion engine, in order to achieve both a reduction in emissions and suppression of a deterioration in drivability in a compatible manner, the fuel injection amount and the ignition timing are optimally controlled in accordance with the alcohol concentration of the fuel.
In this connection, a plurality of kinds of alcohol-mixed fuel that have different alcohol concentrations to each other are commercially available as alcohol-mixed fuels. Consequently, when refueling a vehicle, it is possible that fuel having an alcohol concentration that is different to the alcohol concentration of the fuel that is currently being used will be added to the fuel tank. In this case, although the alcohol concentration in the fuel tank will change in accordance with the refueling amount, the alcohol concentration of fuel that has already been sucked up from the fuel tank before refueling, that is, residual fuel in the fuel supply line, is maintained as it is at the alcohol concentration prior to refueling. As a result, fuel having the same alcohol concentration as prior to refueling is injected from the fuel injection valves for a while after refueling is performed, and thereafter fuel whose alcohol concentration was changed by refueling is injected from the fuel injection valves.
In the conventional FFV internal combustion engine, the alcohol concentration of the fuel is learned based on a feedback correction amount of air-fuel ratio feedback control, and engine control is performed based on the learned alcohol concentration. Consequently, when the alcohol concentration of fuel that is injected from a fuel injection valve changes accompanying refueling, engine control that is based on an unstable alcohol concentration is performed during a period until a switch from the pre-refueling fuel to the post-refueling fuel is completed. Hence, in the conventional FFV internal combustion engine, there is a possibility that emissions will increase and the drivability will deteriorate during a period in which the fuel that is injected from the fuel injection valves is being switched from the pre-refueling fuel to the post-refueling fuel.
The above described problem is particularly noticeable in a dual-injection internal combustion engine as described in the aforementioned patent literature. A dual-injection internal combustion engine is equipped with an in-cylinder injection valve and a port injection valve for each cylinder, and fuel supply lines for distributing fuel to the fuel injection valves of each cylinder are arranged for the fuel injection valve and the port injection valve, respectively. Furthermore, the fuel injection amounts of the two fuel injection valves are not the same, and the injection ratio between the two fuel injection amounts is changed in accordance with the operating state of the internal combustion engine. Therefore, a difference arises between the fuel supply line for the in-cylinder injection valve and the fuel supply line for the port injection valve with respect to the time period until the fuel inside the respective fuel supply lines is completely switched from the pre-refueling fuel to the post-refueling fuel. As a result, in a dual-injection internal combustion engine, a state in which the alcohol concentration of fuel injected from a port injection valve and the alcohol concentration of fuel injected from an in-cylinder injection valve are different to each other continues for a long time period, and because the alcohol concentration of the injected fuel with respect to the internal combustion engine overall is not stable during that time period, an increase in emissions and a deterioration in drivability are liable to occur in comparison to a single-injection internal combustion engine that includes only in-cylinder injection valves or port injection valves.